Vacuum pumps which are operated at high rotor speeds are, for example, turbo vacuum pumps (axial, radial) or molecular pumps, turbomolecular pumps in particular. Molecular pumps, turbomolecular pumps or combinations of these, belong to the class of gas friction pumps. The active pumping surfaces of a molecular pump are formed by the surfaces of a rotor and a starer which face each other, whereby the rotor and/or the stator is equipped with a thread-like structure. Turbomolecular pumps exhibit--much like turbines--rotor and starer blades which form the active pumping surfaces. The active pumping surfaces on the inlet side form the high vacuum (HV) area. The active pumping surfaces close to the ejection side represent the forevacuum (FV) area. In combined gas friction pumps a turbomolecular pump will generally be employed in the HV area whereas a molecular pump will be used in the FV area.
Gas friction pumps of the aforementioned kind are suited for the generation of a high vacuum (from 10.sup.3 mbar and less). They require a backing pump, a rotary vane pump, for example, which is connected to the FV area.
Pumps of the aforementioned kind are being employed more and more for the evacuation of chambers and vessels in which chemical processes such as coating or etching etc. are run. In such applications, relatively large quantities of gas are produced which must be removed by the gas friction pump in order to maintain the required vacuum pressures. Large quantities of gas present a great thermal lead to the active pumping surfaces. Also the drive motor and the rotor bearings--be they rolling bearings or magnetic bearings--contribute to the generation of heat. Also the process gas itself may be warm in the examples given. For these reasons, gas friction pumps employed in connection with chemical processes must be equipped with a cooling system in a manner which is basically known.